1. Field of the Invention
The present invention relates to radio receivers and transmitters for direct sequence spread spectrum communications, and more particularly, to a radio system in which a received carrier signal modulated by a digital code sequence is directly down-converted to a baseband output signal.
2. Description of Related Art
Radio systems utilizing spread spectrum modulation techniques are increasingly popular for communications, navigation, radar and other applications. In a spread spectrum system, the transmitted signal is spread over a frequency band that is wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems have reduced susceptibility to interference or jamming, and enable high data integrity and security. Moreover, by spreading transmission power across a broad bandwidth, power levels at any given frequency within the bandwidth are significantly reduced, thereby allowing such systems to operate outside of certain FCC licensing requirements. In view of these significant advantages, spread spectrum communication systems are highly desirable for commercial data transmission.
In one type of spread spectrum communication system, an RF carrier is modulated by a digital code sequence having a bit rate much higher than that of the information signal. These systems are known as "direct sequence" modulation systems. In a direct sequence spread spectrum system the RF carrier is typically modulated by two data streams in quadrature with each one having one phase when the data stream code sequence represents a "one" and 180.degree. phase shift when the data stream code sequence represents a "zero." Since the digital code sequence comprises a pattern of square wave half-periods that vary in duration, the spectral power envelope of a direct sequence modulated signal is of a [(sin x)/x].sup.2 form. This type of modulation is commonly referred to as Quadrature Phase Shift Key (QPSK) modulation.
The radio receiver recovers the information from the received signal by use of two separate processes. First, the received signal is down-converted from the center frequency, f.sub.c, of its RF carrier to a fixed intermediate frequency to enable processing of the signal. Conventional signal processing techniques, such as heterodyne reception, can be applied to down-convert the received signal. Second, the spreading code modulation is removed or demodulated to reveal the information of the signal. Demodulation of the spreading code modulation is accomplished by multiplication with a code reference sequence identical in structure and synchronized in time with the received signal, a process known as correlation. These techniques may be performed simultaneously.
In conventional heterodyne receiving systems, the received signal is beat against a sine wave generated by a local oscillator having a frequency different from the center frequency of the carrier, f.sub.O. The mixer generates a set of sum and difference frequencies which correspond to the original received signal. Essentially, the mixer performs a frequency conversion, which results in the received signal being converted to a replica of the received signal at an intermediate frequency (IF) comprising the difference between the carrier frequency and the local oscillator frequency (f.sub.c -f.sub.o). This way, the information in the signal can be demodulated at a fixed frequency in an IF stage of the receiver.
A drawback of heterodyne receiving systems is that demodulation at the intermediate frequency requires additional translation mixers and tuned filters to attenuate spurious noise signals that result from the heterodyne down-conversion, adding unnecessary complexity to the receiver circuitry. In addition, the frequency conversion process sometimes allows undesired signals, known as the image frequency, to pass through into the IF signal processing stage. Thus, an important consideration in heterodyne receiver design is rejection of the image frequency components.
A secondary problem relates to systems that both receive and transmit signals. To transmit a signal having the same carrier frequency of the received signal, an oscillator to provide the carrier frequency must be included in the receiving and transmitting system. Since the local oscillator of the heterodyne receiver produces a frequency offset from the carrier frequency, the local oscillator must either be re-tuned for transmission operation, or a second oscillator must be provided. Rapid retuning of the local oscillator is problematic at relatively high transaction rates, and can result in transmission delay. Also, the addition of the second oscillator further increases complexity of the radio system.
Accordingly, a direct sequence spread spectrum radio receiving and transmitting system which avoids the complexity of heterodyne reception would be very desirable.